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Spade: an H Chondrite Impact-Melt Breccia That Experienced Post-Shock Annealing

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Spade: an H Chondrite Impact-Melt Breccia That Experienced Post-Shock Annealing Meteoritics & Planetary Science 38, Nr 10, 1507–1520 (2003) Abstract available online at http://meteoritics.org Spade: An H chondrite impact-melt breccia that experienced post-shock annealing Alan E. RUBIN 1* and Rhian H. JONES 2 1Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095–1567, USA 2Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA *Corresponding author. E-mail: [email protected] (Received 26 January 2003; revision accepted 19 October 2003) Abstract–The low modal abundances of relict chondrules (1.8 vol%) and of coarse (i.e., ³200 mm- size) isolated mafic silicate grains (1.8 vol%) in Spade relative to mean H6 chondrites (11.4 and 9.8 vol%, respectively) show Spade to be a rock that has experienced a significant degree of melting. Various petrographic features (e.g., chromite-plagioclase assemblages, chromite veinlets, silicate darkening) indicate that melting was caused by shock. Plagioclase was melted during the shock event and flowed so that it partially to completely surrounded nearby mafic silicate grains. During crystallization, plagioclase developed igneous zoning. Low-Ca pyroxene that crystallized from the melt (or equilibrated with the melt at high temperatures) acquired relatively high amounts of CaO. Metallic Fe-Ni cooled rapidly below the Fe-Ni solvus and transformed into martensite. Subsequent reheating of the rock caused transformation of martensite into abundant duplex plessite. Ambiguities exist in the shock stage assignment of Spade. The extensive silicate darkening, the occurrence of chromite-plagioclase assemblages, and the impact-melted characteristics of Spade are consistent with shock stage S6. Low shock (stage S2) is indicated by the undulose extinction and lack of planar fractures in olivine. This suggests that Spade reached a maximum prior shock level equivalent to stage S6 and then experienced post-shock annealing (probably to stage S1). These events were followed by a less intense impact that produced the undulose extinction in the olivine, characteristic of shock stage S2. Annealing could have occurred if Spade were emplaced near impact melts beneath the crater floor or deposited in close proximity to hot debris within an ejecta blanket. Spade firmly establishes the case for post-shock annealing. This may have been a common process on OC asteroids. INTRODUCTION blanket. The identification of an OC impact-melt breccia that was shocked and annealed would confirm the reality of Many ordinary chondrites (OC) exhibit evidence of complex thermal processing on OC asteroids. Different impact melting. Many contain impact-melt-rock clasts (e.g., components of the meteorite could record different episodes Plainview, Tysnes Island, Johnson City) that formed from of the rock’s thermal and shock history. small-scale impacts into the host (Fodor and Keil 1976; Spade appears to be an annealed impact-melt breccia. Wilkening 1978; Rubin et al. 1983). Some OC are impact- The meteorite was found as a single 8.9 kg stone at a location melt breccias (e.g., Rose City, Cat Mountain, Chico) that 8.5 km north and 1.6 km east of the town of Spade in Lamb contain large impact-melted regions adjacent to relict County, Texas. chondritic regions (Mason and Wiik 1966; Kring et al. 1996; Bogard et al. 1995). A few OC are impact-melt rocks (e.g., ANALYTICAL PROCEDURES PAT 91501, Y-74160, Y-790964, Ramsdorf) that have been extensively or completely melted (Mittlefehldt and Lindstrom Polished thin section UNM 1099 (~185 mm 2) of Spade 2001; Takeda et al. 1984; Yamaguchi et al. 1998, 1999); they was studied microscopically in transmitted and reflected light. contain little relict chondritic material. Grain sizes were measured with a petrographic microscope Some OC have been shocked and annealed (e.g., Rubin using a calibrated reticle. Modal abundances were determined 2002, Forthcoming). Annealing may have been caused by microscopically using an automated point counter. Polished burial of these samples in proximity to impact melts beneath slab C359.1 (~2500 mm 2) from the collection of the Institute the floor of an impact crater or near hot debris in an ejecta of Meteoritics at the University of New Mexico was examined 1507 © Meteoritical Society, 2003. Printed in USA. 1508 A. E. Rubin and R. H. Jones microscopically. The thin section and slab were etched with a This is much higher than the values for L and LL chondrite dilute solution of nitric acid to reveal metallographic details. falls (8.3 and 3.6 wt%, respectively; Jarosewich 1990). Quantitative analyses of olivine, plagioclase, chromite, Mineral compositions also indicate that Spade is an H- metal, and sulfide were performed using the JEOL 733 group chondrite. The mean olivine and low-Ca pyroxene Superprobe electron microprobe at UNM, with operating compositions in Spade (Fa 19.0 and Fs16.0, respectively; Table conditions of 15 kV and a beam current of 20 nA. Silicate 2) are within the H4–6 chondrite ranges (Fa 16–20, Fs14.5–18; standards were used for silicate minerals, and a combination Brearley and Jones 1998). The mean plagioclase composition of metal and sulfide standards was used for metal and sulfide (n = 24) of Ab 79.9Or4.8 (Table 2) is closest to the mean of H mineral analyses. Low-Ca pyroxene and Ca pyroxene were chondrite plagioclase (Ab 81.9Or5.8; Van Schmus and Ribbe analyzed with the JEOL JXA-8200 electron microprobe at 1968) and appreciably less sodic than the mean compositions UCLA, using natural and synthetic standards, an accelerating of plagioclase in L and LL chondrites (i.e., Ab 84.2Or5.6 and voltage of 15 keV, a 15 nA sample current, 20 sec counting Ab85.9Or3.6, respectively). The mean kamacite Co content of times, and ZAF corrections. Spade (0.46 ± 0.03 wt%; Table 3) is within the H chondrite range (0.44–0.51 wt% Co; Rubin 1990) and the kamacite Ni RESULTS content (6.9 wt%; Table 3) is identical to the mean value in H4–6 chondrites (Rubin 1990). Weathering Grade Texture and Mineral Chemistry Thin section and slab examination indicate that ~60% of the silicate grains exhibit significant brown staining. Many The Spade whole-rock does not appear to be brecciated; mafic silicate grains are traversed and surrounded by 0.1 mm- no clasts are visible in the slab. The rock is rich in metallic Fe- thick veins of limonite ( a-FeO[OH]). The majority of Ni and contains metal veins. One curvilinear metal vein is metallic Fe-Ni grains in the interior appear unaltered; they lack limonite rims and are consistent with weathering stage Table 1. Modal abundances of major phases in Spade. a W1 (Wlotzka 1993). However, within 3–4 mm of the Vol% Wt% Mean H (wt%) b meteorite surface, brown staining is much more extensive and Silicate 84.5 73.5 74.7 most metal and sulfide grains have been largely or completely Troilite 5.4 6.6 5.3 altered. Numerous subparallel 10–100 mm-thick veins and Metallic Fe-Ni 8.7 18.2 18.6 associated veinlets of limonite occur in these extensively Chromite 0.6 0.7 0.6 weathered regions, consistent with weathering stage W5. Limonite 0.8 0.9 – Total 100.0 99.9 100.0 Chondrite Group Nr of points 2190 2190 – Area (mm2) 200 200 – Modal analysis (Table 1) indicates that Spade contains aVol% percent was converted into wt% using the following specific gravities: 18.2 wt% metallic Fe-Ni; this is half way between the silicate, 3.3; troilite, 4.67; metallic Fe-Ni, 7.95; chromite, 4.7; limonite, normative mineralogical value of H chondrites (18.6 wt%; 4.28. bNormative mineralogy from Table 4.1 in Dodd (1981); total includes Table 4.1 in Dodd 1981) and the mean of H chondrite falls 0.6 wt% apatite and 0.2 wt% ilmenite. analyzed by wet chemistry (17.8 wt%; Jarosewich 1990). Table 2. Compositions (wt%) of silicate and oxide phases in Spade. a Olivine Low-Ca pyx Ca pyx Plagioclase Chromite Nr of grains 9 7 10 24 10 SiO2 39.1 ± 0.4 55.7 ± 0.4 53.6 ± 0.3 64.8 ± 2.0 <0.04 TiO2 <0.04 0.27 ± 0.05 0.41 ± 0.06 – 1.9 ± 0.2 Al2O3 <0.04 0.36 ± 0.09 0.99 ± 0.10 22.6 ± 1.5 3.5 ± 0.7 Cr2O3 0.04 ± 0.01 0.57 ± 0.11 1.5 ± 0.12 n.d. 60.6 ± 0.7 V2O3 n.d. n.d. n.d. n.d. 0.67 ± 0.07 FeO 17.9 ± 0.3 10.7 ± 0.1 5.1 ± 0.2 0.55 ± 0.14 27.4 ± 1.1 MnO 0.52 ± 0.02 0.52 ± 0.05 0.29 ± 0.03 n.d. 0.91 ± 0.08 MgO 42.9 ± 0.2 30.2 ± 0.4 17.8 ± 0.2 <0.04 4.8 ± 0.7 CaO <0.04 1.8 ± 0.2 19.3 ± 0.2 3.3 ± 1.7 n.d. Na2O <0.04 0.08 ± 0.03 0.73 ± 0.07 9.4 ± 0.7 n.d. K2O n.d. <0.02 <0.02 0.86 ± 0.48 n.d. Total 100.5 100.2 99.8 101.5 99.8 End member Fa19.0 ± 0.3 Fs16.0 ± 0.1Wo3.5 ± 0.4 Fs8.4 ± 0.3Wo40.2 ± 0.5 Ab79.9 ± 5.7Or4.8 ± 2.7 an.d. = not determined. Spade: An H chondrite impact-melt breccia 1509 Table 3. Compositions (wt%) of opaque phases in Spade. a Modal analysis (2868 points) of the thin section indicates Kamacite Plessite Troilite Pent that Spade contains 1.8 vol% relict barred olivine (BO) and Nr of grains 28 9 18 1 porphyritic olivine-pyroxene (POP) chondrules.
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